In a conventional electrostatographic copying process, a latent electrostatic image is formed on the insulating surface of an element such as a photoconductor. If a dry development process is used, charged toner particles are applied to the electrostatic image, where they adhere in proportion to the electrostatic potential difference between the toner particles and the charges on the latent image. Toner particles that form the developed image are then transferred to a receiver to which the transferred image is fixed, usually by thermal fusion. It is conventional to transfer toner particles from the image bearing element to the receiver by means of an electrostatic bias between the receiver and the element.
While the conventional electrostatic transfer process works well with large toner particles, difficulties arise as the size of the toner particles is reduced. Smaller toner particles are necessary for images of high resolution and low granularity. As the particle size of the toner falls below about 8 micrometers, however, the surface forces holding the toner particles to the element tend to dominate over the electrostatic force that can be applied to the particles to assist their transfer to the receiver. Thus, less toner transfers and image quality suffers. In addition, as the particle size decreases, Coulombic repulsion between the particles tends to scatter the particles, causing loss in image resolution and increase in graininess and mottle. Thus, high resolution images require very small particles, but high resolution images without image defects have not been achievable using electrostatic transfer.
An important advance in the non-electrostatic transfer of toner particles to a receiver surface has been described in the copending U.S. patent application of Rimai et al, Ser. No. 230,394, filed Aug. 9, 1988, now U.S. Pat. No. 4,927,727. The described procedure, which is called thermally assisted transfer, is especially advantageous for toner powders of small particle size, e.g., below about 8 micrometers.
In the thermally assisted transfer process, the receiver, which can be a sheet of paper, is heated, and is pressed against the toner particles on the element. The heated receiver sinters the thermoplastic toner particles, causing them to stick to each other and to the receiver. The element and receiver are then separated and the toner image is fixed, e.g., by ferrotyping to the receiver. For details, see copending application of Rimai et al, Ser. No. 230,394, entitled "Thermally Assisted Transfer of Small Electrostatographic Toner Particles", filed Aug. 9, 1988, which is incorporated herein by reference.
To aid in transferring all of the toner particles from the element to the receiver, it is advantageous to coat the receiving surface of the receiver with a thermoplastic polymer. During transfer the toner particles adhere to or become partially embedded in the thermoplastic coating and are thereby more completely removed from the element. A further improvement in the procedure is to coat the thermoplastic polymer layer on the receiver with a release agent such as zinc stearate. These improvements and preferred materials for the thermoplastic layer and the release agent are disclosed in more detail in the copending application of Rimai et al, Ser. No. 345,160, entitled "Method of Non-Electrostatically Transferring Toner", filed Apr. 28, 1989, now U.S. Pat. No. 4,968,578, and incorporated herein by reference.
While a release agent can advantageously be coated on the thermoplastic layer of the receiver sheet, other techniques can also be used to improve the transfer efficiency. For example, when the binder resin for the photoconductor and the thermoplastic polymer layer of the receiver are appropriately selected with respect to their compositions and surface energies, a release agent is not necessary. Examples of preferred materials are disclosed in copending application of Light et al, Ser. No. 455,673, entitled "Thermally Assisted Transfer of Electrostatographic Toner Particles to a Thermoplastic Bearing Receiver", filed Dec. 22, 1989, which is incorporated herein by reference.
Even with the new thermally assisted transfer process disclosed in the cited copending applications, certain problems relating to the conventionally available receiver sheets are encountered.
Receiver sheets for electrophotographic toner images mainly have been paper although plastic sheets have also been used. Both have disadvantages, especially for receiving, fusible toner powder of small particle size in the making of continuous tone or half-tone electrophotographic reflective prints. To use a plastic sheet for this purpose the plastic must be pigmented, e.g., with titanium dioxide or the like in order to provide an opaque, reflective support for the toner image. Blending a colorant with the polymer adds cost and the pigmented sheet has a higher specific gravity. Furthermore, colorants tend to fade or otherwise change color with aging.
As for paper, its surface is too rough for high resolution of images. The paper must, therefore, have a smooth surface layer of plastic or clay. This, of course, adds cost. A particularly serious disadvantage is that, being fibrous and hydrophilic, paper unavoidably contains moisture. When heated, as in the thermally assisted transfer of toner powder to the receiver sheet and in fusing toner to the receiver sheet, the moisture in the paper vaporizes and causes the paper to buckle and blister in the toned image, especially in large areas of toner.
A need has existed, therefore, for an improved receiver sheet, especially for thermally assisted transfer, but also for other transfer techniques such as electrostatic, when the image has large solid areas of toner. The receiver should have several important properties. First, it should be suitable for the fusion and fixing thereto of toner powders of small particle size to provide images of high resolution. The sheet must retain dimensional stability when heated during the transfer and fixing of toner to it. The sheet must be low in moisture content in order to avoid problems caused by water vaporization during heating. In addition, for receiver sheets having a thermoplastic surface layer there must be good adhesion between the thermoplastic surface layer and the substrate of the sheet in order to avoid delamination when heated. Then, of course, as a support for electrophotographic prints, the sheet must be substantially opaque and highly reflective for visible light. For convenience in handling, the sheet should be flexible and of reasonably low specific gravity. It should also have low manufacturing cost.
The present invention offers further improvement in the forming of images of high resolution by thermally assisted transfer of small particle toners. It provides this improvement by the use of a type of receiver sheet which has not heretofore, been used for toner images. It also provides an improvement regardless of the method of image transfer when the toner images have large solid areas of toner. Images of this kind include continuous tone electrophotographic prints and also half-tone images, particularly when dot spread occurs and creates large solid toner areas, as well as largely alpha-numeric images that in addition have solid areas such as graphics and corporate logos.
A problem with all such images is that the toner in the large solid areas will crack and the paper will deform when paper is the receiver sheet. The reason is that paper absorbs moisture and when the paper dries out it shrinks while the large toner areas do not. The dimensional changes of the paper relative to the toner areas will create an unsightly appearance in the graphics or logos or will damage any continuous tone or half tone images having large toner areas. The paper may also curl or wrinkle. Ordinary plastic sheets, although not moisture-absorbent, also have drawbacks mentioned previously.
The receiver sheet of the present invention provides unexpected advantages over previously used receiver materials for toner images of the types mentioned.